So I was reading over your post, and even though this is older and has to do with CS5, I have some information for CS6 that will be relevant to your question now that it is time to upgade the software. In general Photoshop has always been more about RAM than Processing power, though that is important too. Until CS5 it could never quite take advantage of what multi-core processors really had to offer, and even the graphics card didn't really effect much of it outside of certain features and the 3D optio
Now that is no longer true and Adobe can effeciently use all aspects of the computer to give you much better performance.
So much so that I was able to buy a brand New ASUS with the following specs:
Dual Core 2.2GHz Processors
8GB of RAM (4GB original to the machine)
Windows 7 Home Premium 64Bit OS
320GB HDD w/ 5400RPM
This machine not only runs photoshop CS6 and Launches in less than 5 Seconds... it can run Premier and After Effects as well, and this Laptop cost me less than $400 (on sale at Best Buy 2 Weeks Ago. You don't have to buy something this cheap, but isn't it great that you can and still run Master Collection effectively?
You can read my details on how to buy a Photoshop CS6 Laptop, what specs you need, comparison between Mac and PC and watch the video of my Photoshop Speed Test by reading this article: